Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretched films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing blank as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more layers of unexposed (uncured) photopolymer, a protective layer or slip film, and a cover sheet.
The one or more unexposed photopolymer layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a solid composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If a second photocurable layer is used, i.e., an overcoat layer, it typically is disposed upon the first layer and is similar in composition.
The photopolymer materials generally cross-link (cure) and harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and infrared wavelength regions. Preferred actinic wavelength regions are from about 250 nm to about 450 nm, more preferably from about 300 nm to about 400 nm. One suitable source of actinic radiation is a UV lamp, although other sources are generally known to those skilled in the art.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a “continuous in-the-round” (CITR) photopolymer sleeve. CITR sleeves have applications in the printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
A flexographic printing element is produced from a photopolymer printing blank by imaging the photopolymer printing blank to produce a relief image on the surface of the printing element. This is generally accomplished by selectively exposing the photocurable material to actinic radiation, which exposure acts to harden or crosslink the photocurable material in the irradiated areas. The areas that are not exposed to actinic radiation can then be removed in a subsequent step.
The printing element is selectively exposed to actinic radiation in one of several related ways. In a first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In a second alternative, the unexposed photopolymer layer is coated with an actinic radiation (substantially) opaque layer that is sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. This technique is well-known in the art, and is described for example in U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, and in U.S. Pat. No. 5,925,500 to Yang et al., the subject matter of each of which is herein incorporated by reference in their entirety. In a third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods is acceptable, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer.
Any conventional sources of actinic radiation can be used for the exposure step. Suitable visible or UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units and photographic flood lamps, by way of example and not limitation.
After imaging, the photosensitive printing element is processed or “developed” to remove uncured (i.e., non-crosslinked) portions of the photopolymer layer, without disturbing the cured portions of the photopolymer layer, to produce the relief image on the surface of the printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include thermal development or the use of an air knife.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly. Thus, processes have been developed whereby photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 7,122,295, 6,773,859, 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. U.S. 2006/0124009 and, U.S. 2010/0119978, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for use of the process in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
The composition of the photopolymer is such that there exists a substantial difference in the behavior of the cured and uncured polymer when subjected to heat. It is precisely this difference that allows the creation of an image in the cured photopolymer when heated. The uncured photopolymer (i.e., the portions of the photopolymer layer not contacted with actinic radiation) melts or substantially softens while the cured photopolymer remains solid and intact at the temperature chosen for thermal processing. Thus the difference in behavior allows the uncured photopolymer to be selectively removed, thereby creating an image.
The printing element is heated to a temperature sufficient to effect melting or softening by conduction, convection or other heating method as is known in the art. For example, the printing element may be heated to a temperature of at least about 70° C., more typically between about 120 to about 200° C. The exact temperature will depend upon the properties of the particular photopolymer being used. However, two primary factors are generally considered in determining the development temperature:                1) The development temperature is preferably set between the melt or softening temperature of the uncured photopolymer on the low end and the melt or softening temperature of the cured photopolymer on the upper end. This will allow selective removal of the photopolymer, thereby creating the image; and        2) The higher the development temperature, the quicker the process time will be. However, the development temperature should not be so high as to degrade the cured photopolymer. The temperature should be sufficient to melt or substantially soften the uncured photopolymer, thereby allowing it to be removed.        
Once the printing element has been heated, uncured photopolymer can be melted or removed. The heated printing element is contacted with a material that will absorb or otherwise remove the softened or melted uncured photopolymer. This removal process is generally referred to as “blotting” and is typically accomplished using an absorbent web of material. Either woven or non-woven material can be used and the material can be polymer based or paper, so long as the material is capable of withstanding the operating temperatures involved. Blotting is accomplished using one or more rollers to bring the blotting material and the heated printing plate element into contact.
The uncured photopolymer layer is heated by conduction, convection, or other heating method to a temperature sufficient to effect melting. By maintaining more or less intimate contact of the absorbent sheet material with the photocurable layer, a transfer of the uncured photopolymer from the photopolymer layer to the absorbent sheet material takes place. While still in the heated condition, the absorbent sheet material is separated from the cured photopolymer layer in contact with the support layer to reveal the relief structure. After cooling, the resulting flexographic printing plate can be mounted on a printing plate cylinder.
Upon completion of the development step, the printing plate element is optionally, but preferably, post-exposed to further actinic radiation and/or detacked. The printing element may then be cooled and is ready for use.
A typical apparatus for thermally development (also known as thermal processing) comprises:                a) Means for supporting the flexographic printing element;        b) Heating means for softening or melting non-crosslinked photopolymer on the imaged and exposed surface of the flexographic printing element;        c) At least one roll that is capable of bringing a blotting material into contact with the surface of the flexographic printing element to remove the softened or melted non-crosslinked photopolymer on the surface of the flexographic printing element; and        d) Means for maintaining contact between the at least one roll and the surface of the flexographic printing element.        
U.S. Pat. Pub. No. 2010/0119978 to Vest and U.S. Pat. Pub. No. 2006/0124009 to Markhart, the subject matter of each of which is herein incorporated by reference in its entirety describe thermal development apparatuses in which the printing element is heated to a temperature sufficient to selectively melt or soften the uncured portions of the at least one layer of photopolymer such that the softened or melted uncured photopolymer is removable from the printing element by contacting the heated printing element with a blotting material.
One problem that can arise in thermal processing is that the blotting material may not carry away all of the uncured photopolymer. Various methods have been previously been used for preventing buildup of uncured photopolymer material on the surface of the hot roll. For example, the hot roll may be covered with a non-stick coating to prevent uncured photopolymer from adhering to the hot roll, the hot roll may be subjected to periodic reverse cycles of rotation against a stationary blotter, or the hot roll may be manually cleaned by mechanical cleaning (i.e., abrasives), use of a chemical cleaning solution, or both. However, the use of periodic reverse cycles of rotation and manual cleaning of the hot roll both require that the thermal processor be taken off-line for cleaning. Furthermore, if the hot roll is covered with a non-stick coating, the non-stick coating may eventually wear off, especially at the elevated temperatures of the thermal processor, and cease working.
Thus, there remains a need in to the art for an improved thermal development processor that includes an improved means for cleaning the hot roll and that overcomes the deficiencies of the prior art.